I have the same issue and have posted a thread as well. Love to know if there is a related bug post to this issue.
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i was doing the part where you kill the Sword of Oryx with my Sol-Edge with a friend. After turning it in i got both the Void and Solar quest parts. Problem solved. i had upgrade the void-edge on my Titan while he still had the quest from the Sol-Edge, so the quest steps over-lapped. So now the issue is actually getting void kills with a freaking Titan.